1. Field of the Invention
The present invention relates generally to energy conversion processes. More particularly, the invention pertains to methods for converting ligno-cellulosic materials resistant to biotreatment into materials which are highly amenable to biotreatment processes for forming energetic fuels in gaseous and/or liquid states.
2. State of the Art
In a long-standing wastewater treatment method commonly known as anaerobic digestion, anaerobic life forms convert a portion of the “volatile” materials in municipal wastewater sludge into a digester gas. Typically, less than about 50 percent of the volatile material in the sludge is converted to digester gas, a useful fuel largely comprised of methane (CH4) and carbon dioxide (CO2), typically in about a 63:35 proportion. Inasmuch as the ultimate goal is complete elimination of the volatiles in the sludge, a large portion of the volatile solids is disposed of in some other way, e.g. burning or landfill. This presents special and unique problems in California and other states that impose de-facto bans against thermal processes (e.g. combustion) For waste-to-energy (WTE) conversion.
The definition of “volatiles” used in wastewater treatment is found in Standard Methods 2540 G, wherein the “volatiles” analysis comprises the solids loss upon volatilization and combustion of a solids sample ignited at 500° C.
To those skilled in the art, it is apparent that anaerobic digestion should be theoretically applicable to the treatment of many other materials including for example, animal wastes, fats, oils, greases, plant materials, municipal solid wastes (MSW), wood, low-rank coal, lignite, and the like. While anaerobic digestion has been generally successful for hydrocarbon materials, i.e. oils and greases, such treatment of ligno-cellulosic materials has resulted in very slow and incomplete conversion. Because of the huge supply of such materials found in wastes and renewable resources, the need for methods to convert such materials into useable energetic products at high yield is readily evident. Such methods would also greatly reduce the need for landfilling of solid waste materials as is done currently.
More recently, efforts at producing methane from specific raw materials by utilizing various treatment steps (including anaerobic digestion) has resulted in the use of the term “biomethanation” to broadly describe such processes.
It is generally known that a substantial portion of the volatile materials in sewage sludge which are resistant to biomethanation typically comprise cellulosic lignins. These materials are very recalcitrant to biodegradation in part because they have a very low water-solubility.
Another disadvantage of conventional biomethanation is the low rate at which many non-lignic components become dissolved to become amenable to biological conversion. In the treatment of municipal wastewater sludges for example, this necessitates the use of very large digesters, as is well known in the waste treatment field. Yet, about one-half or more of the volatiles remain unconverted and must be disposed of in some other way. A method for enabling complete rapid conversion of volatile materials in the digester would be extremely beneficial in several ways. First, the amount of sludge solids which must be ultimately disposed of by landfill or out-of-state waste-to-energy facilities is reduced. Secondly, the quantity of digester gas with valuable heating value is simultaneously increased. A further possible benefit is a reduction in the required digestion residence time, which will increase the capacity of in-place digestion equipment.
A prior art method for producing a fuel gas from carbonaceous materials, e.g. coal, lignite, peat, etc. is known as high temperature gasification, wherein an oxygen-containing gas is used for burning a portion of the input materials to achieve the necessary minimum gasification temperature of 1500° F.-1600° F. (815° C.-871° C.). The produced syngas typically contains quantities of carbon monoxide CO and hydrogen H2, but a major portion will be carbon dioxide CO2. Water H2O is also produced. Inasmuch as a significant portion of the produced energetic liquid is water soluble, the water produced in the gasification reactions negatively affects the yield, unless energy is expended to dewater the water-soluble materials. Furthermore, a significant portion of the input carbonaceous material is consumed to produce the high reactor temperature, and cannot be recovered as a fuel. Thus, the efficiency of converting the carbon-containing material to energetic fuel is lower than desired.
The term “volatiles” as used in the fields of combustion and pyrolysis is defined in ASTM D3172: Proximate Analysis. The analytical method comprises drying of the solids and heating in an airtight crucible to a temperature of approximately 1,700° F. (927° C.). The weight loss represents the volatile portion. References to “volatiles” in the remainder of this discussion will use the combustion/pyrolysis definition thereof, i.e. pyrolysis at 1,700° F.
A number of firms, indicated at:    http://www.coskata.com/ and http://www.ineosbio.com/57-Welcome to INEOS Bio.htm are reportedly gasifying waste materials at high temperatures to produce a syngas that has significant fractions of carbon monoxide (CO), hydrogen (H2), and water. After separating out the other components of the syngas, the carbon monoxide, hydrogen and water are fed into a bioreactor where proprietary microorganisms initiate a bacterial fermentation process and eventually produce ethanol. There are a number of issues associated with this process. For example, the ethanol producing organisms are not considered robust.
Pyrolysis is a treatment method in which a substance is changed by subject to heat alone. Pyrolysis differs from gasification in that there is an absence of oxygen. Pyrolysis has been long used for making charcoal from wood. Typically, about 25 to 35 percent of the carbon in wood becomes carbonized, i.e. fixed as charcoal, and the remainder of the wood is converted to gaseous products, e.g. carbon dioxide, carbon monoxide, methane, and hydrogen; and condensable substances including various alcohols, organic acids, ammonia, ketones, phenols, creosote, oils, tars, and water. The manufacture of charcoal has long been accomplished using relatively uncontrolled batch processes. The results have varied, depending upon the heating rate and time, maximum temperature, type of wood, size of wood particles, and other factors.
Since the oil crisis of the 1970's, a number of researchers have attempted pilot scale pyrolysis designed to produce a “bio-oil” from various forms of biomass. High temperature pyrolysis differs from “gasification”. In conventional pyrolysis, oxygen is substantially excluded and the operating temperatures to achieve destructive distillation are much lower, typically about 750° F.-800° F. (399° C.-427° C.). The destructive distillation results in a gas, a liquid and solid matter, i.e. char. The combined water/organic liquid product that is condensed out after the pyrolysis or destructive distillation step is known as “pyroligneous acid”. The goal of the “bio-oil” process is to eventually produce an oily product that can be burned in a diesel engine. These small plants have suffered and continue to suffer from a number of operational problems. Although under the proper time, temperature and heating rate conditions, the pyroligneous acid may include a phase resembling an “oil-like” substance, it is oxygenated and the oils and other components are partially soluble in the significant quantities of water that are also produced in pyrolysis. The bio-oil is reported to be very corrosive. The heating value of the oily product is relatively low.
Approximately 200 chemical species have been identified in the liquid product from a pyrolysis conversion of biomass to wood vinegar and wood alcohol. The primary species include alcohols such as methanol, butanol, amyl alcohol, etc.; acids such as acetic, formic, propionic, valeric, etc.; bases such as ammonia, methylamine, pyridine, etc.; phenol and phenol-like substances syringol, cresol, etc.; and neutral substances such as formaldehyde, acetone, furfural, valerolactone, etc.
The solid byproducts remaining from pyrolysis of wood are sometimes termed “biochar” or “Terra Preta”, and have been found to have agronomically beneficial properties.
Another variant of pyrolysis, generally known as terrefaction, was traditionally used for roasting coffee beans. It involves heating a biomass material to about 200-320° C. (392-608° F.) in an oxygen-free atmosphere for about 30 to 90 minutes. About 70-80 percent of the starting material is converted to a dense solid.
Despite the problems in directly producing a liquid fuel, nearly all current research in pyrolytic fuel production continues to be directed at producing a “bio-oil” for diesel engines from a woody biomass. For example, in U.S. Pat. No. 5,959,167 to Shabtai et al., lignin is converted to oxygenated gasoline compositions by a process including a catalyzed depolymerization, followed by a selective catalytic hydrocracking and an exhaustive etherification reaction.
In U.S. Pat. No. 7,578,927 to Marker et al., substances simulating gasoline and diesel oil are produced by subjecting cellulosic waste to pyrolysis to form a liquid stream and a lignin stream. The separated lignin stream is subjected to a hydrotreating (partial cracking) step at 500-3000 psia to decarboxylate the lignin into oils.
Other approaches to converting biomass to fuels are extant. For example, in U.S. Pat. No. 7,494,637 to Peters et al. and U.S. Pat. No. 5,865,898 to Holtzapple et al., biomass is mixed with a metal oxide and reacted at 1400° C. or higher temperature to form biomass carbides. The product is then quenched to less than 800° C. to form gaseous acetylene.
One approach to resolving the problem created by lignin blockage of celloulosic surfaces is a pre-hydrolysis step to initially dissolve the linkage between lignin and hemicellulose. In U.S. Pat. Nos. 4,880,473; 5,395,455 and 5,605,551 to Scott et al., a process is disclosed for making fermentable sugars from wood which includes a first hydrolysis with sulfuric acid at elevated temperature to dissolve hemicellulose while leaving most cellulose as a solid. The acidified solid phase is then subjected to a very short “flash” pyrolysis of less than 2 seconds at temperatures of 400-600 ° C. The pyrolysis product contains sugars and anhydro-sugars as well as lignin materials. Water is added and the insoluble lignin-containing materials are separated from the soluble aqueous phase containing the fermentable sugars. The method is relatively expensive. Similar hydrolysis methods are disclosed in U.S. Pat. Nos. 5,424,417; 6,002,419, and 6,228,177 of Torget, et al.
In U.S. Pat. No. 7,608,439 to McTavish et al., oxygen-free combustion gases containing carbon dioxide CO2 are fed to an anaerobic digester for conversion of CO2 to methane CH4.
A careful reading of the following description in correlation with the appended drawings of the invention will define the differences of the present invention from prior art processes, and will demonstrate the advantages which are attained thereby.